The present invention relates to a pressure sensor for determination of pressure in a combustion chamber of internal combustion engines.
Pressure sensors of the above mentioned general type are known in the art, for example from German document DE-OS 3,125,640.6. In this pressure sensor the piezoresistive measuring elements, such as for example thick film resistors of cermet, contactive plastic or metal are applied on a support. The resistance element and the support are arranged as close as possible to the pressure chamber to determine the pressure which acts in it. Furthermore, the measuring signal is supplied by electrical conductors to an electronic evaluating circuit arranged outside the housing of the pressure sensor. For this purpose the piezoresistive elements and the electronic structural parts must be connected with one another by means of screened conduits which are quite expensive. Since the piezoresistive measuring element is directly subjected to the pressure, it is also subjected to high temperatures which act in the combustion chamber. The flame expands there with a temperature of approximately 2,000.degree. C., and pre-tensioning in the housing can occur. Thereby the pressure signal is distorted due to the high temperatures.
Another pressure sensor is disclosed in European patent document EO-OS 85,111,895.0. Here the thick film resistor is arranged on the bottom of a support. This pressure sensor is however provided only for the determination of the pressure in distributing pumps. The high temperature which acts in the combustion chamber would also distort the measuring signal.
U.S. Pat. No. 4,645,965 describes a pressure sensor in which the measuring element is composed of a piezoelectric material. This piezoelectric element is arranged in a housing which has an opening facing the combustion chamber. The opening is closed by a membrane, and the piezoelectric element is in operative communication through a plunger with a membrane and thereby the pressure acting in the combustion chamber. The piezoelectric element abuts against a shoulder in the housing. Due to the use of the piezoelectric element structural differences are produced. In general it is required to operate the measuring element in a potential-free manner. Thereby it is required, in addition to both necessary contact discs for the piezoelectric element, to provide a further isolating disc between the contact disc and the counter bearing. The contacting of the piezoelectric ceramic discs is possible only by expensive welding of wires with the contact discs or by making a cavity in the plunger. Since the piezoelectric element must be used as a discrete structural part it is relatively expensive. Furthermore, several expensive adhesive locations are required, which also distort the measuring signal. In the normal situation four adhesive points are needed for a piezoelectric disc and two contact discs. In a potential-free sensor five adhesive points are needed. Due to the measuring principle used here, the plunger must be glued perpendicularly on the piezoelectric element. Piezoelectric elements have a disadvantageous, relative temperature course of the measuring sensitivity which can be within 15% and 70% of the utilized temperature region. The aging over the service life amounts to typically to 2%-10%. The hysteresis of the piezoceramic of 5-25% is very high and limits the accuracy of the combustion chamber measurements. In particular the multi-layer structure of the piezoelectric arrangement requires a relatively expensive and complicated insertion in the housing of the pressure sensor. With the piezoelectric elements, in contrast to the piezoresistive elements, during the action of pressure for producing the measuring signal, a charge and thereby a voltage is produced. This voltage is detected and evaluated. In contrast, with the piezoresistive elements a voltage is provided, and the electrical resistance in the piezoresistive element is changed due to the pressure which acts in this case.